Field of the Invention
The invention relates to a reception diversity method and to a radio communications system using diversity reception. In a reception diversity method a first antenna receives a first signal and a second antenna receives a second signal. The first and second signals are then converted from an analog format to a digital format and the resulting digital signal values are detected and quality levels are in each case defined for the digital signal values. Such a radio communications system is formed of at least two base stations each having an antenna for receiving the first and second signals. In addition, analog-to-digital converters are provided for converting the analog signals along with a device for detecting the resultant digital signal.
In radio communications systems, information (for example voice, video information, internet messages or other data) are transmitted using electromagnetic waves via a radio interface between a transmitting and a receiving radio station (base station and mobile station, respectively). The electromagnetic waves are in this case transmitted using carrier frequencies that are in a frequency band intended for the respective system.
Known second-generation digital mobile radio systems are the global system for mobile communications (GMS) mobile radio system and the IS-95 mobile radio system, which use carrier frequencies of 900, 1800 and 1900 MHz. Frequencies in the frequency band around 2000 MHz are envisaged for future mobile radio systems using CDMA or TDMA/CDMA transmission methods via the radio interface, for example universal mobile telecommunication system (UMTS) or other 3rd-generation systems.
Reception diversity methods are known from digital radio communications systems, for example Published, European Patent Application EP 96 114 902 A in this context, discloses received signals which have been received at different points and are combined after being converted from analog to digital form and after detection in order to improve the reception quality. Two methods are known for combination, they are selective combination and maximum ratio combining (MRC) combination. In the former case, the signal whose quality is better is selected. The quality of signals is normally assessed by using quality levels related to a frame and obtained in the receiver.
In the other case, the signals are linked on the basis of individual symbols and their quality levels, and a sum signal is formed. The data rates that need to be transmitted for MRC combination of the received signals and quality levels are, however, considerable, so that greatly derated transmission capacities need to be reserved on the network side.
It is accordingly an object of the invention to provide a reception diversity method, and a radio communication system using diversity reception which overcome the above-mentioned disadvantages of the prior art methods and devices of this general type, which reduces the data rates that needs to be transmitted with high reception quality.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for reception diversity, which includes:
receiving a first received signal via a first antenna;
receiving a second received signal via a second antenna;
converting the first received signal and the second received signal from an analog format to a digital format resulting in digital signal values;
detecting the digital signal values;
defining quality levels for the digital signal values; and
evaluating in an evaluator the quality levels, and requesting only portions of the digital signal values in dependence on results of an evaluation.
In the radio diversity method according to the invention, the first received signal is received by the first antenna, and the second received signal is received by the second antenna. The first received signal and the second received signal are converted from an analog format to a digital format, and digital signal values as well as quality levels related to them are defined in a digital receiver. Quality levels are advantageously defined not only for each of the digital signal values, but also for each frame. At least a portion of the quality levels is evaluated by an evaluation device and, depending on the evaluation result, only portions of the digital signal values are requested for subsequent combination.
In consequence, only those signal values and quality levels that are actually required for diversity combination need be transmitted. If selective combination can be used relatively frequently, then the data rate that needs to be transmitted is considerably reduced. If the line capacity between the evaluation device and the antenna and/or receiver is severely limited, at least partial MRC combination is thus feasible for the first time, since the multiple data rate need not be reserved for all connections.
The process of passing on the digital signal values is requested adaptively by an evaluation device. The method results in advantages particularly if the antennas are widely separated from the device carrying out the combination, for example in the case of macro-diversity reception. The data rate can be matched precisely to the transmission capacity of the lines, which are often only rented.
According to one advantageous development of the invention, only the first digital signal values are requested for passing on if the quality level for the first digital signal values is considerably better than the quality level for the second digital signal values. In this case, the results of selective combination are better, that is to say the selection of the first digital signal values and the transmission of the second digital signal values in the network can be stopped at an early stage.
If the quality levels for the first digital signal values are only slightly better than the quality levels for the second digital signal values, at least portions of both digital signal values are passed on with the associated quality levels, for example on a symbol basis. MRC combination can thus be used which, in this case, provides values whose quality is better.
In order to reduce the data rate further, only particularly important symbols are passed on for MRC combination. The gain from MRC combination is at its greatest for transmission quality in the case of these particularly important symbols.
Furthermore, it is advantageous to monitor the change in the quality levels over a time period that can be predetermined and, in the event of rapid changes, to prefer that at least portions of both digital signal values be passed on. This reduces the signaling complexity and the delay time, since there is no need to wait for data that can be requested from time to time.
The reduction in the data rate that needs to be transmitted is particularly important if the two antennas are disposed in different base stations in a radio communications system, and the evaluation device is disposed in a base station controller which forms digital signal values that result from the two digital signal values. The base station and the base station controller are two components in the radio communications system, which communicate with one another for data transmission. The data rate of this connection is reduced by the invention. With regard to hand over procedures for a connection between two base stations, the so-called xe2x80x9csoft hand overxe2x80x9d with two reception paths at times, the adaptive control process according to the invention for signal transmission matched to the requirement results in considerable advantages.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a reception diversity method, and a radio communication system using diversity reception, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.